Mercury recovery apparatus

ABSTRACT

Both ends of a bulb of a waste fluorescent lamp are cut off, a phosphor layer formed on an interior surface of the bulb is detached, so that mercury-containing phosphor powder can be obtained. The mercury-containing phosphor powder is subjected to a heating and reducing process with the dry-method, by mixing an organic reducing agent with the phosphor powder and heating the mixture, to vaporize and separate mercury from the phosphor powder. The vaporized mercury is then cooled and condensed, to collect mercury.

RELATED APPLICATION

This application is a divisional application of U.S. Ser. No. 10/109,126filed on Mar. 28, 2002 now U.S. Pat. No. 6,800,112.

BACKGROUND OF THE INVENTION

(1) Field of the Invention

The present invention relates to a mercury recovery method and a mercuryrecovery apparatus, and particularly to a technique for separating andcollecting mercury contained in phosphors used in fluorescent lamps.

(2) Related Art

Mercury, which is a toxic material, is used in lighting devices such asfluorescent lamps. When fluorescent lamps whose life is expired orfluorescent lamps that are found defective during manufacturingprocesses (hereafter simply referred to as “waste fluorescent lamps”)are disposed, mercury is usually to be recovered from the wastefluorescent lamps. The recovered mercury is processed to removeimpurities etc., and then is recycled.

As a method for recovering mercury from phosphor powder used in wastefluorescent lamps, the wet-method is conventionally known, one exampleof which is disclosed in Japanese laid-open patent application No.H10-12149. According to the disclosed method, mercury-containingphosphor powder detached from a waste fluorescent lamp is immersed in anaqueous solution in which active sulfur is dissolved, to form a mercuricsulfide compound that is refractory, so that mercury can be recoveredfrom the phosphor powder.

Also, as another method for recovering mercury from phosphor powder, thedry-method is conventionally known, one example of which is disclosed inJapanese published examined patent application No. S53-1594. This methodtakes advantage of a characteristic of mercury as being a low-boilingmetallic element. According to the disclosed method, mercury-containingphosphor powder is subjected to a heating process to vaporize mercurycontained therein, and the vaporized mercury is then cooled andcondensed, so that mercury can be recovered from the phosphor powder.

Here, rare-earth phosphors, which are expensive, are often used influorescent lamps for general lighting. When waste fluorescent lamps aredisposed, therefore, it is desirable to recycle not only mercury, butalso phosphor powder from which mercury is separated (hereafter simplyreferred to as “treated phosphor powder”).

However, with the above-described conventional wet-method, a portion ofthe phosphor powder is dissolved into the aqueous solution containingactive sulfur, with the crystal structure of phosphors being destroyedor changed. This causes characteristics of the phosphor powder includingluminance to deteriorate, making recycling of the treated phosphorpowder impossible.

Further, unlike the dry-method mercury recovery, the wet-method mercuryrecovery generally requires special equipment for liquid-wastetreatment, including treatment of used aqueous solutions etc. Therefore,the wet-method mercury recovery tends to suffer from high-cost, comparedwith the dry-method mercury recovery.

With the conventional dry-method mercury recovery, substantially allmercury, when being contained in the form of metallic mercury ormercurous oxide, can be separated and collected from phosphor powder.However, the problem is that phosphor powder of life-expired fluorescentlamps, in particular, contain mercury in the form of an amalgam that isformed by reacting with an emitter (emissive material) or a phosphorelement. It is difficult to decompose an amalgam by heat, andaccordingly it is difficult to separate and collect substantially allmercury from such phosphor powder. To solve this problem, increasing aheating temperature in the heating process may be considered. However,the temperature being too high in the heating process causes thermaldegradation of phosphor powder, which results in characteristics of thephosphor powder including luminance deteriorating, making recycling ofthe treated phosphor powder impossible.

SUMMARY OF THE INVENTION

In view of the above problems, a first objective of the presentinvention is to provide a mercury recovery method that ensures recoveryof mercury in any forms, such as metallic mercury, from phosphor powder,and that enables the treated phosphor powder to be recycled, withoutrequiring special equipment for liquid-waste treatment. A secondobjective of the present invention is to provide a mercury recoveryapparatus that produces the same effect as the mercury recovery method.

The first objective of the present invention can be achieved by amercury recovery method for recovering mercury from mercury-containingphosphor powder that has been detached from a fluorescent lamp, themercury recovery method including the steps of: vaporizing mercurycontained in the phosphor powder by subjecting the phosphor powder to aheating and reducing process; and condensing the vaporized mercury bycooling, to collect the vaporized mercury.

According to this method, a heating process is carried out in a reducingatmosphere. Therefore, mercury contained in phosphor powder in anyforms, such as metallic mercury, mercurous oxide, and an amalgam formedby reacting with an emitter or a phosphor element, can be reduced in theheating process. Therefore, atomization of the mercury in any forms canbe facilitated. This can ensure vaporization and separation ofsubstantially all mercury contained in the phosphor powder even atrelatively low temperatures. In particular, because the mercury can bevaporized and separated at relatively low temperatures, thermaldegradation of the phosphor powder can be reduced. As a result,characteristics of the phosphor powder including luminance can beprevented from deteriorating, enabling the treated phosphor powder to berecycled. Further, because the mercury recovery is achieved with thedry-method, special equipment for liquid-waste treatment that is usuallyrequired by the wet-method mercury recovery is not required.

The second objective of the present invention can be achieved by amercury recovery apparatus that recovers mercury from mercury-containingphosphor powder, including: a reaction vessel; a distillation vesselthat is set in the reaction vessel and in which the mercury-containingphosphor powder is placed; a heating unit for heating the distillationvessel; and a mercury collection unit for condensing vaporized mercuryby cooling, to collect the vaporized mercury, the vaporized mercuryhaving been vaporized from the mercury-containing phosphor powder in thedistillation vessel heated by the heating unit, wherein the distillationvessel includes a heat conductive member that is placed so as to come incontact with a part of an interior wall of the distillation vessel, theheat conductive member being for transmitting heat to middle portions ofthe phosphor powder within the distillation vessel.

According to this construction, the entire portions of phosphor powderplaced in the distillation vessel can be heated uniformly. This canfacilitate vaporization and separation of mercury from the phosphorpowder. Therefore, the temperature of middle portions of the phosphorpowder in the distillation vessel can be increased to a temperaturenecessary for thermal decomposition of mercury compounds, withoutexcessively increasing a heating temperature by the heating unit. Also,because the phosphor powder is not heated up to excessively hightemperatures, thermal degradation of the phosphor power can be reduced.As a result, characteristics of the phosphor powder including luminancecan be prevented from deteriorating, enabling the treated phosphorpowder to be recycled. Also, because the mercury recovery is achievedwith the dry-method, special equipment for liquid-waste treatment thatis usually required by the wet-method mercury recovery is not required.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other objects, advantages and features of the invention willbecome apparent from the following description thereof taken inconjunction with the accompanying drawings that illustrate a specificembodiment of the invention.

In the drawings:

FIG. 1 is a partially cutaway view of a mercury recovery apparatus thatis used to realize a mercury recovery method relating to a firstembodiment of the present invention;

FIG. 2 shows a state where both ends of a fluorescent lamp are cut off;

FIG. 3 is a diagram for explaining a process for detaching a phosphorlayer from the fluorescent lamp whose both ends are cut off;

FIG. 4 is a table showing experimental results of mercury recovery withthe mercury recovery method relating to the first embodiment;

FIG. 5 is a table showing measured results of relative luminance ofphosphor powder for practical examples using the mercury recovery methodrelating to the first embodiment and for comparative examples;

FIG. 6 is a graph showing the relationship between an amount of organicreducing agent (wt %) to be added to phosphor powder and an amount ofresidual mercury in treated phosphor powder (%);

FIG. 7A shows an appearance of a distillation vessel in a mercuryrecovery apparatus relating to a second embodiment of the presentinvention;

FIG. 7B schematically shows heat transmission paths when thedistillation vessel is used;

FIG. 8 is a table showing experimental results of mercury recovery bythe mercury recovery apparatus relating to the second embodiment;

FIG. 9 is a plan view of the distillation vessel in which a dividingmember of another example is provided; and

FIG. 10 shows an appearance of the distillation vessel in which adividing member of another example is provided.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following describes preferred embodiments of the present invention,with reference to the drawings.

(First Embodiment)

FIG. 1 shows the construction of a mercury recovery apparatus 100 thatis used to realize a mercury recovery method according to the firstembodiment of the present invention.

As shown in the figure, the mercury recovery apparatus 100 includes acylindrical reaction vessel 1 that is made of metal and can behermetically sealed, a cylindrical distillation vessel 3 that is set onthe bottom of the reaction vessel 1 and into which a mixture 2 ofmercury-containing phosphor powder and an organic reducing agent is tobe placed, a heating apparatus 4 that heats the mixture 2 placed in thedistillation vessel 3 to vaporize and separate mercury from the phosphorpowder, and a mercury collection unit 5 that pumps in the mercury vaporfrom the reaction vessel 1, and cools and condenses the mercury vapor tocollect the mercury.

The distillation vessel 3 is made of a stainless steel and is detachablefrom a bottom la of the reaction vessel 1. Also, a pipe 6 for supplyinggases, such as nitrogen and air, into the reaction vessel 1 is connectedto a lower side surface of the reaction vessel 1. A pipe 7 fordischarging the gases from the reaction vessel 1 into the mercurycollection unit 5 is connected to an upper side surface of the reactionvessel 1.

The heating apparatus 4 includes, for example, a heater 41 that iscoiled up around the exterior surface of the reaction vessel 1, and apower supply circuit 42 for supplying power to the heater 41. Within thereaction vessel 1, a temperature sensor (not shown) such as athermocouple is provided. Based on a value detected by the temperaturesensor, the power supply circuit 42 controls power supply to the heater41 so as to heat up the mixture 2 to a predetermined temperature, andalso to keep an atmospheric temperature within the reaction vessel 1 atthe predetermined temperature.

The mercury collection unit 5 includes a mercury condensing unit 8 forcooling and condensing vaporized mercury, a suction pump 9 for pumpingthe vaporized mercury from the reaction vessel 1 into the mercurycondensing unit 8, and a mercury collection vessel 10 in which thecondensed mercury is collected.

As one example of the suction pump 9, a rotary pump is used. In thepresent embodiment, the mercury condensing unit 8 is of water-coolingtype. A pump (not shown) circulates cooled water in an internallyequipped metallic pipe 8 a. This water circulation cools the mercuryvapor down to approximately 0° C. and thereby condenses the mercuryvapor. The condensed mercury is collected in the mercury collectionvessel 10 via a pipe 8 b.

The following describes a method for detaching phosphor powder from awaste fluorescent lamp, and separating and collecting mercury from thedetached phosphor powder using the above-described mercury recoveryapparatus 100.

As FIG. 2 shows, both ends including end caps 131 of a waste fluorescentlamp 13 in which mercury is enclosed are cut off.

As FIG. 3 shows, an air nozzle 14 is inserted into one opening end of abulb 12 of the waste fluorescent lamp 13 whose both ends have been cutoff, and an aspirator 15 is attached to the other opening end of thebulb 12. Air with a predetermined pressure is insufflated onto theinterior surface of the bulb 12 from the air nozzle 14, so as to detacha phosphor layer 11. The detached phosphor layer 11 is aspirated by theaspirator 15 in a state of being powdered. The aspirator 15 includes anenclosure 15 b internally equipped with a high-density filter 15 a, anda vacuum pump 15 c for depressurizing the inside of the envelope 15 b toa negative pressure. The phosphor powder that has been detached by airblow from the air nozzle 14 and aspirated into the envelope 15 b isfiltered through a filter 15 a. The phosphor powder is gradually sievedout, and is collected into the distillation vessel 3 placed below theenvelope 15 b.

Following this, an aqueous solution of an organic reducing agent whoseconstituents include carbon, oxygen, and hydrogen that are unreactive tophosphor powder is added into the distillation vessel 3. The aqueoussolution and the phosphor powder are stirred and mixed thoroughly, togenerate a mixed solution (the mixture 2 in FIG. 1). The reason formixing the organic reducing agent dissolved for example in water withthe phosphor powder is to uniformly mix the phosphor powder and theorganic reducing agent and to uniformly cause atomization of mercury inthe entire phosphor powder. The atomized mercury has a lower boilingpoint, and therefore, can be vaporized and separated from the phosphorpowder when heated at relatively low temperatures.

Note here that the constituents of the organic reducing agent may not belimited to the above-listed carbon, oxygen, and hydrogen, but mayfurther include other elements that are unreactive to phosphor powdersuch as nitrogen. Note also that, as the organic reducing agent, it ispreferable to select at least one from the group consisting of ascorbicacid, oxalic acid, formic acid, maleic acid, citric acid, malonic acid,stearic acid, phthalic acid, tartaric acid, succinic acid, and sulfamicacid.

Following this, as FIG. 1 shows, the distillation vessel 3, in which themixture 2 of the phosphor powder and the organic reducing agent isplaced, is set within the reaction vessel 1. Then, power is supplied tothe heater 41 of the heating apparatus 4, to heat the mixture 2 in thedistillation vessel 3 via the reaction vessel 1. In this way, thedetached phosphor powder is subjected to a heating and reducing processwith the dry-method.

This heating and reducing process causes the following chemicalreaction. Atomization occurs to mercury in any forms contained in thephosphor powder, such as metallic mercury, mercurous oxide, and anamalgam formed by reacting with an emitter or a phosphor element. Morespecifically, carbon monoxide, methane, and hydrogen, which aredecomposition products of the reducing agent, reduce the mercury in anyforms so as to have a valence of zero. The atomized mercury is thenvaporized and separated from the phosphor powder.

Here, an internal pressure of the reaction vessel 1 may be anatmospheric pressure. In view of preventing thermal oxidization of thephosphor powder, however, it is more preferable to reduce the internalpressure (to 20000 Pa for example) with the use of the suction pump 9.Also, an internal atmosphere of the reaction vessel 1 may be an airatmosphere. In view of further preventing thermal oxidization of thephosphor powder, however, it is more preferable to introduce a nitrogenatmosphere in the reaction vessel 1. In this case, a nitrogen gascylinder is connected to the pipe 6 via a pressure reducer or the like.

Also, a heating temperature of the mixture 2 is set equal to or higherthan such a temperature that can thermally decompose at least mercury inany forms contained in the phosphor powder. For example, when onlymetallic mercury and mercurous oxide are contained in the phosphorpowder, the heating temperature is set equal to or higher than 450° C.When an amalgam is contained in the phosphor powder in addition tothese, the mixture 2 is to be heated up to approximately 600° C.

The vaporized mercury within the reaction vessel 1 is pumped by thesuction force of the suction pump 9 into the mercury condensing unit 8via the pipe 7. In the mercury condensing unit 8, the mercury vapor iscooled and condensed, resulting in mercury drops being collected intothe mercury collection vessel 10.

Gases other then mercury pumped into the mercury condensing unit 8 bythe suction pump 9, which include for example decomposition products ofthe reducing agent, are discharged outside via a pipe 16. Here, theproduced gases are mostly carbon dioxide and water vapor, which areharmless to be released outside without any treatment. Accordingly,special equipment for disposing such gases is not required.

The mercury collected in the mercury collection vessel 10 is furtherprocessed by industrial experts to remove impurities, and then isrecycled.

On the other hand, the phosphor powder remaining in the distillationvessel 3 is put through a sieve (not shown) to remove glass pieces etc.that are mixed therein when the phosphor layer 11 formed on the interiorsurface of the bulb 12 is detached. The sieved phosphor powder isprocessed with a well-known air classification apparatus (not shown) tofurther remove fine impurities, resulting in the phosphor powdercontaining no impurities. The phosphor powder containing no impuritiesis washed and dried, and then is recycled.

The following sums up the above-described mercury recovery method forseparating and collecting mercury from phosphor powder in thefluorescent lamp 13 equipped with the bulb 12 on the interior surface ofwhich a phosphor layer made of phosphor powder is formed. The bulb 12 isfirst cut off, and then the phosphor layer 11 is detached from theinterior surface of the bulb 12 to obtain mercury-containing phosphorpowder. The obtained phosphor powder is subjected to the heating andreducing process with the dry-method, to vaporize and separate mercuryfrom the phosphor powder. Then, the vaporized mercury is cooled andcondensed, to collect the mercury. According to this method, the heatingand reducing process causes the atomization reaction of mercury in anyforms such as metallic mercury, mercurous oxide, and an amalgam formedby mercury reacting with an emitter or a phosphor element. Morespecifically, the reducing agent reduces the mercury of any forms so asto have a valence of zero. Therefore, it is ensured that substantiallyall mercury contained in the phosphor powder can be vaporized andseparated from the phosphor powder at relatively low temperatures. Inparticular, because mercury can be vaporized and separated from thephosphor powder at relatively low temperatures due to the action ofreducing agent, thermal degradation of the phosphor powder can bereduced, thereby preventing characteristics of the phosphor powderincluding luminance from deteriorating.

This method enables treated phosphor powder that has conventionally beendisposed without any choices to be recycled, and so is extremelyeconomical. Further, because this method employs the dry-method, specialequipment for liquid-waste treatment that is usually required by thewet-method mercury recovery is not required.

Also, this method uses an organic reducing agent whose constituents arecarbon, oxygen, and hydrogen, the decomposition products of the organicreducing agent including carbon monoxide, methane, and hydrogen etc.These decomposition products do not react with the phosphor powder, andtherefore, do not remain in the phosphor powder.

EXPERIMENTAL EXAMPLES

The following describes experimental examples that can verify the effectof the mercury recovery method relating to the first embodiment.

First, phosphor powder was detached from a waste straight-tubefluorescent lamp 13 with rated power of 40 W whose life has beenexpired. Phosphors used in this waste fluorescent lamp 13 are rare-earthphosphors with the following compositions:

Red phosphor Y₂O₃:Eu Blue phosphor (SrCaBa)₅(PO₄)₃Cl:Eu Green phosphorLaPO₄:Tb, Ce

An amount of mercury contained in the detached phosphor powder, that is,phosphor powder from which mercury was yet to be separated (hereaftersimply referred to as “untreated phosphor powder”) was 2800 μg perphosphor powder 2 g.

To prepare a mixture A (practical example 1), the untreated phosphorpowder was mixed with an aqueous solution in which oxalic acid was beingdissolved, at a rate of oxalic acid 0.4 g (20 wt %) to phosphor powder 2g.

To prepare a mixture B (practical example 2), the untreated phosphorpowder was mixed with an aqueous solution in which ascorbic acid wasbeing dissolved, at a rate of ascorbic acid 0.4 g (20 wt %) to phosphorpowder 2 g.

The mixture A and the mixture B were placed in separate distillationvessels 3. As to each of the mixtures A and B, mercury was separated andcollected from the phosphor powder using the mercury recovery methodrelating to the present embodiment under the conditions of: heatingtemperature 600° C.; heating time 30 min.; an atmospheric pressure; andan air atmosphere. Then, a residual rate (%) of mercury contained in thetreated phosphor powder with respect to mercury contained in theuntreated phosphor powder was measured. The experimental results areshown in Table 1 in FIG. 4.

Comparative experiments (comparative examples 1 and 2) were carried outas follows. For the comparative example 1, the detached phosphor powderwas directly placed in a distillation vessel 3 without adding anyreducing agent. Then, mercury was separated and collected from thephosphor powder using the same method and under the same conditions asin the above practical examples 1 and 2. A residual rate (%) of mercurycontained in the treated phosphor powder with respect to mercurycontained in the untreated phosphor powder was measured. For thecomparative example 2, the detached phosphor powder was directly placedin a distillation vessel 3 without adding any reducing agent. Then,mercury was separated and collected from the phosphor powder with thesame method and under the same conditions as in the above practicalexamples 1 and 2 except that the heating temperature was set at 800° C.for the comparative example 2. A residual rate (%) of mercury containedin the treated phosphor powder with respect to mercury contained in theuntreated phosphor powder was measured. The experimental results of thecomparative examples 1 and 2 are also shown in Table 1.

Note here that the residual rate of mercury (%) was measured in thefollowing way. The phosphor powder, from which mercury was separated,was dissolved into aqua regia, and then an amount of mercury in theresulting solution was measured using hydride generation atomicabsorption spectrometry. Note also that an air inflow to the reactionvessel 1 was set at 21/min. in each of the practical examples and thecomparative examples.

As Table 1 shows, the residual rate of mercury is 0.12% for thepractical example 1, 0.14% for the practical example 2, and 0.12% forthe comparative example 2. On the other hand, the residual rate ofmercury is 0.30% for the comparative example 1.

The reason for the results implying that the residual rate of mercury ishigher for the comparative example 1 than for the practical examples 1and 2 and the comparative example 2 can be considered as follows. Theheating temperature of the heater 41 being approximately 600° C. is nothigh enough to decompose an amalgam formed by mercury reacting with anemitter or a phosphor element that is hard to decompose by heat. Thereason for this is considered that a temperature of middle portions ofthe phosphor powder placed in the distillation vessel 3 does not reach600° C. For the comparative example 1, therefore, mercury contained inthe phosphor powder in the form of such an amalgam remains in thephosphor powder. On the other hand, for the comparative example 2, theheating temperature of the heater 41 being 800° C. can decompose mercurycontained in the form of such an amalgam that is difficult to decomposeby the heating temperature being approximately 600° C. As for thepractical examples 1 and 2, the residual rate of mercury is low, despitethe heating temperature of the heater 41 being approximately 600° C. Thereason for this can be considered that vaporization and separation ofmercury in any forms including an amalgam formed by reacting with anemitter or a phosphor element are facilitated because the heatingprocess is carried out in an reducing atmosphere.

Next, for the practical examples 1 and 2 and the comparative example 2,a relative luminance (%) of the treated phosphor powder to a luminanceof the untreated phosphor powder (assumed to be 100%) was examined. Theexperimental results are shown in Table 2 in FIG. 5.

As Table 2 shows, the relative luminance is 90% for the practicalexamples 1 and 2, and 75% for the comparative example 2. The reason forthis can be considered as follows. For the practical examples 1 and 2,the heating temperature of phosphor powder is 600° C., which isrelatively low, and therefore, thermal degradation of the phosphorpowder can be reduced. On the other hand, for the comparative example 2,the heating temperature of phosphor powder is 800° C., which is high,and therefore, thermal degradation of the phosphor powder occurs.

The above experiments verify the following. As described above, for thepractical examples 1 and 2, it is ensured that substantially all mercuryin any forms including metallic mercury can be separated and collectedfrom the phosphor powder. Further, thermal degradation of the phosphorpowder can be reduced, and therefore, the treated phosphor powder can berecycled.

It should be noted here that although an amount of ascorbic acid and anamount of oxalic acid are each set at 20 wt % to the phosphor powder inthe above practical examples 1 and 2, a rate of these organic reducingagents to the phosphor powder is not limited to such a value.

FIG. 6 is a graph showing the experimental results relating to therelationship between (a) the amount of ascorbic acid/oxalic acid addedto the phosphor powder and (b) the amount of mercury remaining in thetreated phosphor powder. The horizontal axis indicates the amount oforganic reducing agent (wt %) to be added to the phosphor powder, andthe vertical axis indicates the amount of mercury (%) remaining in thetreated phosphor powder with respect to the amount of mercury containedin the untreated phosphor powder.

As the graph shows, with the amount of ascorbic acid/oxalic acid beingless than 10 wt % to the phosphor powder, the residual mercury amount isrelatively high. With the amount of ascorbic acid/oxalic acid beingabout 10 wt % to the phosphor powder, the residual mercury amount dropsdrastically. With the amount of ascorbic acid/oxalic acid being in arange of 10 to 20 wt % to the phosphor powder, the residual mercuryamount decreases only slightly. With the amount of ascorbic acid/oxalicacid being more than 20 wt % to the phosphor powder, the residualmercury amount hardly decreases.

These experimental results imply that it is preferable to set the amountof organic reducing agent at 10 wt % or more with respect to phosphorpowder. These experimental results also imply that an excessively largeamount of organic reducing agent does not produce more favorablereducing action but results in the phosphor powder being discoloredbrown due to its residual materials, and accordingly, an excessivelylarge amount of organic reducing agent rather hinders the process ofenabling the phosphor powder to be recycled. In conclusion, anexcessively large amount of organic reducing agent does not produce theeffect of facilitating vaporization and separation of mercury, butrather produces the adverse effect of discoloring the phosphor powder.To be more specific, there seems to be no reason to add the organicreducing agent by 20 wt % or more to the phosphor powder, because withthe amount of organic reducing agent being more than 20 wt % to thephosphor powder, the residual mercury amount hardly decreases. It shouldbe noted here that discoloration of the phosphor powder was not observedwith the amount of organic reducing agent being 20 wt % to the phosphorpowder.

As can be known from the above, it is preferable to set the amount oforganic reducing agent in a range of 10 to 20 wt % inclusive withrespect to the phosphor powder.

Also, although the present embodiment describes the case where oxalicacid or ascorbic acid is used as the organic reducing agent,substantially the same effect as produced above can be produced when atleast one organic reducing agent selected from the group consisting offormic acid, maleic acid, citric acid, malonic acid, stearic acid,phthalic acid, tartaric acid, succinic acid, and sulfamic acid, is used.

(Second Embodiment)

The following descries a mercury recovery apparatus according to asecond embodiment of the present invention. This mercury recoveryapparatus has the same construction as the mercury recovery apparatus100 (FIG. 1) used to realize the mercury recovery method in the firstembodiment, except that a dividing member 17 made of a heat conductivematerial is provided in the distillation vessel 3. Accordingly, thepresent embodiment is described focusing only on the distillation vessel3.

FIG. 7A is a perspective view showing one example of the distillationvessel 3 in the second embodiment. As shown in the figure, threestainless cylindrical members each having a different diameter that formthe dividing member 17 are placed substantially concentrically withinthe distillation vessel 3. FIG. 7B schematically shows the state whereheat is transmitted to phosphor powder 20 within the distillation vessel3 in the reaction vessel 1. For ease of explanation, the figure shows avertical section of the distillation vessel 3 etc. in FIG. 7A.

Arrows in FIG. 7B each roughly indicate a heat transmission path. Asshown in the figure, heat applied to the reaction vessel 1 by the heater41 is directly applied to the entire portions of the phosphor powder 20including middle portions in the distillation vessel 3, via the bottom 1a of the reaction vessel 1, the bottom of the distillation vessel 3, andthe dividing member 17. Also, radioactive heat from the interior wall ofthe reaction vessel 1 is easily transmitted to the phosphor powder viathe dividing member 17. This enables the entire portions of the phosphorpowder 2 to be heated at substantially uniform temperatures.

Using this mercury recovery apparatus equipped with the distillationvessel 3 in which the dividing member 17 is provided (practical example4), mercury was separated and collected from phosphor powder to which noorganic reducing agent was added, using the same method and under thesame conditions as for the comparative example 1. A residual amount(mg/l) of mercury contained in the treated phosphor powder was measured.The experimental results are shown in Table 3 in FIG. 8.

It should be noted here that the residual amount of mercury wascalculated based on the elution test method set forth in NotificationNo. 13 of the Environment Agency of Japan. For comparison purposes,residual amounts of mercury for the practical example 1 and thecomparative example 1 in Table 1 in FIG. 4 in the first embodiment werealso calculated based on the same elution test method, and are shown inTable 3.

As Table 3 shows, the residual amount of mercury is 0.0005 mg/l for thepractical example 4, which is the same for the practical example 1. Onthe other hand, the residual amount of mercury is 0.0020 mg/l for thecomparative example 1. The reason for the results implying that theresidual amount of mercury for the practical example 4, where an organicreducing agent is not added to phosphor powder, i.e., where not aheating and reducing process but a heating process is involved, is thesame as the residual amount of mercury for the practical example 1,where a heating and reducing process is involved, can be considered asfollows. The entire portions of the phosphor powder are heated uniformlyat 600° C. via the heat-conductive dividing member 17 provided in thedistillation vessel 3, thereby facilitating vaporization and separationof mercury from the phosphor powder.

As described above, the mercury recovery apparatus in the presentembodiment has an extremely simple construction where a dividing memberis additionally provided in a reaction vessel 1 in a conventionalmercury recovery apparatus, but has the effect of uniformly heating theentire portions of phosphor powder and facilitating vaporization andseparation of mercury from the phosphor powder. This mercury recoveryapparatus can ensure vaporization and separation of substantially allmercury contained in the phosphor powder at relatively low temperatures.

In particular, because mercury can be vaporized and separated from thephosphor powder at relatively low temperatures, thermal degradation ofthe phosphor powder can be reduced. Accordingly, characteristics of thephosphor powder including luminance can be prevented from deteriorating.This enables the treated phosphor powder to be recycled. Also, themercury recovery is achieved with the dry-method, and therefore, specialequipment for liquid-waste treatment that is usually required by thewet-method mercury recovery is not required.

It should be noted here that although the present embodiment describesthe case where three cylindrical members forming the dividing member 17are substantially concentrically placed in the distillation vessel 3,the number of cylindrical members is of course not limited to three.Further, a dividing member 17 a formed by plate members may be providedradially that the plate members of the dividing member 17 a intersectthe cylindrical members of the dividing member 17 as indicated by a planview of the distillation vessel 3 shown in FIG. 9. In this case, an areaof the interior wall of the distillation vessel 3 that comes in contactwith the dividing members is expanded, enabling heat to be transmittedmore easily to the phosphor powder inside the vessel. Therefore, theentire portions of the phosphor powder can be heated more uniformly.

The dividing member 17 may be fixed inside the distillation vessel 3 inadvance, or may be constructed to be detachable from the distillationvessel 3.

Also, the dividing member may take other forms. For example, a dividingmember 17 c that is formed by arranging, in a lattice, plate memberswith high heat-conductivity such as stainless members may be provided inthe distillation vessel 3 as shown in FIG. 10. The internal space of thedistillation vessel 3 may not be divided completely as the case may be.For example, a mass of thermal conductive material placed at the centeror the like of the distillation vessel 3 can produce the effect ofuniformly heating the phosphor powder to a certain degree.

Also, although the above embodiments describe the case where thedistillation vessel 3 is made of a stainless steel, vessels made ofother materials that are unreactive to an organic reducing agent, suchas alumina and graphite carbon, may instead be used.

Also, as a material for a dividing member, ceramics, quartz glass, andthe like may be used depending on the case, instead of theabove-mentioned materials with high heat-conductivity such as astainless steel. Ceramics or quarts glass may not have a higherheat-conductivity than phosphors. However, ceramics or quarts glass isadvantageous because it does not react with phosphors, and has high heatresistance. Moreover, it is considered that phosphors in the powder formhave extremely low heat-conductivity because of a lot of gaps betweenthe particles. Therefore, the heat-conductivity of a dividing membermade of ceramics or quarts is higher than the heat-conductivity of thephosphor powder. Accordingly, the dividing member made of ceramics orquarts can still produce the effect of uniformly heating the phosphorpowder.

Also, although the above embodiments describe the case where mercury isrecovered from rare-earth phosphor powder, the present invention isapplicable in various other cases, for example, a case where mercury isrecovered from halo-phosphate luminescent material powder.

Although the present invention has been fully described by way ofexamples with reference to the accompanying drawings, it is to be notedthat various changes and modifications will be apparent to those skilledin the art. Therefore, unless such changes and modifications depart fromthe scope of the present invention, they should be construed as beingincluded therein.

1. A mercury recovery apparatus that recovers mercury frommercury-containing phosphor powder, comprising: a reaction vessel; adistillation vessel that is set in the reaction vessel and in which themercury-containing phosphor powder is placed; heating means for heatingthe distillation vessel; and mercury collection means for condensingvaporized mercury by cooling, to collect the vaporized mercury, thevaporized mercury having been vaporized from the mercury-containingphosphor powder in the distillation vessel heated by the heating means,wherein the distillation vessel includes a heat conductive member thatis placed so as to come in contact with a part of an interior wall ofthe distillation vessel, the heat conductive member being fortransmitting heat to middle portions of the phosphor powder within thedistillation vessel.
 2. The mercury recovery apparatus of claim 1,wherein the heat conductive member is unreactive to phosphors and ismade of a material having a higher heat-conductivity than the phosphorpowder.
 3. The mercury recovery apparatus of claim 2, wherein thematerial of the heat conductive member is a stainless steel.
 4. Themercury recovery apparatus of claim 1, wherein the heat conductivemember is formed by a plurality of dividing plates that divide aninternal space of the distillation vessel.
 5. The mercury recoveryapparatus of claim 1, wherein the heat conductive material is formed byconcentrically placing a plurality of dividing members that arecylindrical and that each have a different internal diameter, in a statewhere one end of each dividing member comes in contact with a bottomsurface of the distillation vessel.